Picking the appropriate arguments for forwards

If automatic signature reporting doesn’t work for your use case and you still want to specify how a function’s *args, **kwargs is being used, you may either use sigtools.specifiers.forger_function and sigtools.support.s to override its signature completely, or you can use the forwards_to_* functions from sigtools.specifiers.

For forwards_to_* decorators, you only need to specify what function *args, **kwargs are forwarded to and what other arguments are passed in. Here’s a primer with common examples on how to use them.

Picking the appropriate forwards_to_* decorator

Several forwards_to_* decorators exist. You must pick one depending on what you are forwarding your parameters to:

forwards_to_function

When forwarding to a plain function:

def inner(a, b, c):
    ...

@specifiers.forwards_to_function(inner)
def outer(*args, **kwargs):
    inner(*args, **kwargs)

Provide the inner function as the first argument to forwards_to_function.

forwards_to_method

When forwarding to an attribute of the current object, usually to a method:

class Spam:
    def inner(self, a, b, c):
        ...

    @specifiers.forwards_to_method('inner')
    def outer(self, *args, **kwargs):
        self.inner(*args, **kwargs)

Provide the inner function’s name to forwards_to_method.

You can also specify a “deep” attribute: 'attribute.method' should be specified if a call is made like this:

self.attribute.method(*args, **kwargs)
forwards_to_super

When forwarding to the superclass’s method of the same name:

class Spam:
    def method(self):
        pass

class Ham(Spam):
    @specifiers.forwards_to_super()
    def method(self, *args, ham, **kwargs):
        super().method(*args, **kwargs)

This only works when using the short form of super() introduced in Python 3.3. If you are targetting earlier versions, use apply_forwards_to_super on the class, while specifying which methods need to receive the decorator.

For the following examples, we will be using the forwards_to_function decorator, although these will work with other forwards_to_* decorators.

*args and **kwargs are forwarded directly if present

You do not need to signal anything about the wrapper function’s parameters:

@specifiers.forwards_to_function(wrapped)
def outer(arg, *args, **kwargs):
    inner(*args, **kwargs)

This holds true even if you omit one of *args and **kwargs:

@specifiers.forwards_to_function(wrapped)
def outer(**kwargs):
    inner(**kwargs)

Passing positional arguments to the wrapped function

Indicate the number of arguments you are passing to the wrapped function:

@specifiers.forwards_to_function(wrapped, 1)
def outer(*args, **kwargs):
    inner('abc', *args, **kwargs)

This applies even if the argument comes from the wrapper:

@specifiers.forwards_to_function(wrapped, 1)
def outer(arg, *args, **kwargs):
    inner(arg, *args, **kwargs)

Passing named arguments to from the wrapper

Pass the names of the arguments after num_args:

@specifiers.forwards_to_function(wrapped, 0, 'arg')
def outer(*args, **kwargs):
    inner(*args, arg='abc', **kwargs)

Once again, the same goes for if that argument comes from the outer function’s:

@specifiers.forwards_to_function(wrapped, 0, 'arg')
def outer(*args, arg, **kwargs): # py 3
    inner(*args, arg=arg, **kwargs)

If you combine positional and named arguments, follow the previous advice as well:

@specifiers.forwards_to_function(wrapped, 2, 'alpha', 'beta')
def outer(two, *args, beta, **kwargs):
    inner(one, two=two, *args, alpha='abc', beta=beta, **kwargs)

When the outer function uses *args or **kwargs but doesn’t forward them to the inner function

Pass use_varargs=False if your outer function has an *args-like parameter but doesn’t use it on the inner function directly:

@specifiers.forwards_to_function(wrapped, use_varargs=False)
def outer(*args, **kwargs):
    inner(**kwargs)

Pass use_varkwargs=False if you outer function has a **kwargs-like parameter but doesn’t use it on the inner function directly:

@specifiers.forwards_to_function(wrapped, use_varkwargs=False)
def outer(*args, **kwargs):
    inner(*args)

When the outer function passes an arbitrary *args or **kwargs to the inner function

Pass hide_args=True if your outer function uses an arbitrary *args when calling the inner function (whether one exists or not in the outer function):

@specifiers.forwards_to_function(wrapped, hide_args=True)
def outer(**kwargs):
    args = other_function(...)
    inner(*args, **kwargs)

If you know exactly how many items args will have, specify the amount of items in args instead, as in Passing positional arguments to the wrapped function.

Conversely, pass hide_kwargs=True if your outer function uses an arbitrary *kwargs when calling the inner function (whether one exists or not in the outer function):

@specifiers.forwards_to_function(wrapped, hide_args=True)
def outer(*args):
    kwargs = other_function(...)
    inner(*args, **kwargs)

If you know exactly which keys kwargs will potentially have, specify all possible named keys it might have, as in Passing named arguments to from the wrapper.

Note

Neither are needed if the outer function hasn’t got an *args nor **kwargs parameter

Summary

Finally, here’s an overview of all parameters from forwards_to_* functions

sigtools.specifiers.forwards_to_function(wrapped, num_args=0, *named_args, emulate=None, hide_args=False, hide_kwargs=False, use_varargs=True, use_varkwargs=True, partial=False)[source]
num_args
The number of arguments you pass by position, excluding *args.
*named_args
The names of the arguments you pass by name, excluding **kwargs.
use_varargs=
Tells if the wrapper’s *args is being passed to the wrapped function.
use_varkwargs=
Tells if the wrapper’s **kwargs is being passed to the wrapped function.
hide_args=
Tells if the wrapped function is given an *args parameter (other than the wrapper function’s) in such a way that all positional parameters are consumed.
hide_varargs=
Tells if the wrapped function is given a **kargs parameter (other than the wrapper function’s) in such a way that all keyword parameters are consumed.